This invention relates to apparatus and methods for delivering medication to the respiratory system of a patient, preferably an infant, through a pressure-assisted breathing system. More specifically, one aspect of the invention is directed to apparatus and methods for coupling a flow sensor with a continuous positive airway pressure (“CPAP”) system that employs a nebulizer, preferably one having a vibrating aperture-type aerosol generator, to deliver aerosolized medicament simultaneously with CPAP treatment.
The use of CPAP systems and therapies are conventional forms of ventilation treatment for respiratory disorders in both adults and children. In particular, it has been reported that respiratory support with nasal CPAP (“NCPAP”), coupled with simultaneous treatment with nebulized drugs, preferably surfactants, has several advantages in the treatment of infant respiratory distress syndrome (“iRDS”) in pre-term infants (“neonates”). For example, early application of NCPAP and early treatment with aerosolized surfactant in neonates with iRDS have been found to be effective in decreasing the need for mechanical ventilation, with its accompanying mechanical and infectious risks and pathophysiological effects. See, for example, “To the Editor: Surfactant Aerosol Treatment of Respiratory Distress Syndrome in Spontaneously Breathing Premature Infants”; Pediatric Pulmonology 24:22-224 (1997); “Early Use of Surfactant, NCPAP Improves Outcomes in Infant Respiratory Distress Syndrome”; Pediatrics 2004; 11;e560-e563 (as reported online by Medscape Medical News group, Jun. 4, 2004); and “Nebulization of Drugs in a Nasal CPAP System”; Acta Pediatric 88: 89-92 (1999).
CPAP systems utilize a constant positive pressure during inhalation to increase and maintain lung volumes and to decrease the work by a patient during spontaneous breathing. The positive pressure effectively dilates the airway and prevents its collapse. The delivery of positive airway pressure is accomplished through the use of a positive air flow source (“flow generator”) that provides oxygen or a gas containing oxygen through a flexible tube connected to a patient interface device such as nasal prongs (cannula), nasopharyngeal tubes or prongs, an endotracheal tube, mask, etc. CPAP systems typically maintain and control continuous positive airway pressure by using a restrictive air outlet device, e.g. a fixed orifice or threshold resistor, or a pressure valve, which modulates the amount of gas leaving the circuit to which the patient interface device is attached. This pressure regulating device may be placed at, before or beyond the patient interface device and defines a primary pressure-generating circuit.
During the course of conventional CPAP therapy, the patient may typically inhale only a fraction of the total flow of gas passing through the primary pressure-generating circuit. For example, it has been estimated that a CPAP gas flow of 8 L/min may typically result in a pharyngeal tube flow of about 2/L min. As a result, only 25% of aerosolized medicament introduced into the CPAP flow will enter the pharynx. In addition, from this 25% entering the pharynx, about two-thirds may be lost during expiration, assuming an inspiratory/expiratory ratio of 1:2. Thus, in conventional CPAP systems, only 10% of the nebulized drug may enter the patient interface device. This waste, particularly with extremely expensive surfactants, makes the cost of administering nebulized drugs through conventional CPAP systems unacceptably high for routine clinical use. To reduce these costs, the prior art has identified the need for improvements in the method of delivery for aerosolized drugs, e.g. it has been suggested that a method and apparatus are needed for restricting nebulization to inspiration only. See, for example, the article in Pediatric Pulmonology, supra.
It is therefore desirable to find ways to decrease the losses of aerosol particles within pressure-assisted breathing systems during the exhalation phase of the respiratory cycle. In particular, increasing the efficiency in the delivery of aerosolized medicaments through CPAP systems, and the resulting smaller amounts of medicament required for a treatment, can represent a substantial advantage, particularly when scarce and expensive medicaments are employed.